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is the carrying of one or more offspring, known as a fetus or embryo,
inside the womb of a female. In a pregnancy, there can be multiple
gestations, as in the case of twins or triplets. Human pregnancy is the
most studied of all mammalian pregnancies. Childbirth usually occurs
about 38 weeks after conception; in women who have a menstrual cycle
length of four weeks, this is approximately 40 weeks from the last
normal menstrual period (LNMP). The World Health Organization defines
normal term for delivery as between 37 weeks and 42 weeks.
Although pregnancy begins with implantation, the process leading to
pregnancy occurs earlier as the result of the female gamete, or oocyte,
merging with the male gamete, spermatozoon. In medicine this process is
referred to as fertilization; in lay terms, it is more commonly known as
"conception". After the point of fertilization, the fused product of the
female and male gamete is referred to as a zygote or fertilized egg. The
fusion of male and female gametes usually occurs following the act of
sexual intercourse, resulting in spontaneous pregnancy. However, the
advent of artificial insemination and in vitro fertilization have also
made achieving pregnancy possible in cases where sexual intercourse does
not result in fertilization (e.g., through choice or male/female
The process of fertilization occurs in more than a single step, and the
interruption of any of these can lead to a failure. Therefore,
fertilization is much more than the fusion between the female gamete and
male spermatozoon. Through fertilization, the egg and sperm are saved:
the egg is activated to begin its developmental program, and the haploid
nuclei of the two gametes come together to form the genome of a new
Prenatal defines the period occurring "around the time of birth",
specifically from 22 completed weeks (154 days) of gestation (the time
when birth weight is normally 500 g) to 7 completed days after birth.
The postnatal period begins immediately after the birth of a child and
then extends for about six weeks. During this period, the mother's body
returns to pre-pregnancy conditions as far as uterus size and hormone
levels are concerned.
The perinatal period is immediately before to after birth. Depending on
the definition, it starts between the 20th to 28th week of gestation and
ends between 1 to 4 weeks after birth (the word "perinatal" is a hybrid
of the Greek "peri-" meaning 'around or about' and "natal" from the
Latin "natus" meaning "birth.").